1. Field of the Invention
The present invention relates to convergence correcting device in a CRT display apparatus (including an LCLV: Liquid Crystal Light Valve) using a cathode ray tube (called CRT hereinafter) having a two-dimensional display screen.
2. Description of the Related Art
Examples of the convergence correcting device as described above have been disclosed in Japanese Patent Laid-Open No.64-73882, Japanese Patent Laid-Open No.3-22784, and Japanese Patent Laid-Open No.6-133324.
In the conventional type of convergence correcting device as described above, V- and H-convergence correction signals are generated according to V- and H-synchronization data obtained from a composite video signal including not only video information to be regenerated, but also information concerning synchronization in both the vertical (V) and horizontal (H) directions, and convergence correction is executed by driving convergence coils in the V and H sides of a CRT on the basis of the correction signals.
In recent years, there have been developed CRT display apparatus compatible with the so-called multi-scan facility, in which not only a television signal based on the NTSC system but also a high resolution video signal for computer software can be displayed on the same CRT display screen to satisfy the needs such as availability in multimedia. The CRY display apparatus have been disclosed, for instance, in Japanese Patent Publication No.61-6828, or in Japanese Patent Laid-Open No.61-96874.
In the CRT display apparatus compatible with the multi-scan facility as described above, in addition to the composite video signal based on the general NTSC system, various types of composite signals are used, and a position in the time axis to the H-sync. or V-sync. video data signal is not always constant. If, for this reason, a convergence correction signal including a parabola wave is generated on a time axis based on a sync signal, the convergence correction can not always be executed correctly.
For this reason, as disclosed by the applicant for the present invention in Japanese Patent Application No.6-130596, there has been developed a convergence correcting device base on a system in which each point of a first transmission and a last transmission of a parabola wave in a convergence correction signal coincides with each point of a last transmission and a first transmission of either a V-blanking pulse or a H-blanking pulse.
FIG. 1A shows how a voltage P for a V-side convergence correction signal obtained by the convergence correcting device is changed on a central line C-C' on the CRT display screen D of the CRT. Namely, the voltage P for the V-side convergence signal is a parabola wave which becomes maximum at the upper edge as well as at the lower edge of the display screen D and becomes minimum at the central portion thereof in the vertical direction. The maximum and minimum amplitudes in the voltage P are adjusted by an operator checking those on an actual screen according to a sequence of the convergence adjustment. In a Japanese Patent Application No.6-130596, an adjusting circuit for the convergence corrected voltage as described above is disclosed in FIG. 4 of the accompanying drawings thereof and in the detailed description corresponding thereto in the specification thereof.
The trailing and leading edges 1 and 2 of the V-blanking pulse corresponding to a start point and an end point of the parabola wave for the voltage P in the convergence correction signal in FIG. 1A coincide with the upper edge and the lower edge of the display screen D, so that desired convergence correction can be achieved even if there appear changes in a position on the time axis of each V-sync video signal having a different scan speed respectively.
By the way, in a CRT display unit for a personal computer or the like, a raster size of the displayed picture is sometimes different from the raster size which is the expected raster size for the original video signal to be displayed, but only one portion of the video picture represented by the video signal may be displayed with various raster sizes. Herein, the term of "raster size" is defined as the maximum scanning scale on the CRT display screen.
For instance, as shown in FIG. 1B, in case where a size of the V-deflection in the CRT display unit (namely a raster size) is made larger, and an original video signal is extended in the V-direction to display one portion of the original video signal in an enlarged scale in the V-direction, the raster size RS extends, as shown in the figure, to outside of the screen of the display screen D on the CRT. In this case, it is obvious that a start point 3 and an end point 4 of the parabola wave P' in the V-side convergence signal corresponding to the points of the trailing edge of an earlier V-blanking pulse and the leading edge of the succeeding one are positioned respectively before and behind on the time axis the start point and end point 1 and 2 of the parabola wave P corresponding to the reference raster size which matches the size of the display screen D. Even in the case where the parabola wave extends on the time axis as described above, the amplitude of the parabola wave P' at the start and end points 3 and 4 thereof are the same as those adjusted when the reference raster size is previously employed for the reference raster size, so that the amplitude of the parabola wave becomes smaller than that in a case where an amplitude of a parabola wave has a standard raster size at the time points 1 and 2 each corresponding to the upper edge and lower edge of the display screen D.
For this reason, as shown in FIG. 1B, in a case where
the raster size is enlarged, it is required that characteristics in the convergence correction voltage generating circuit are adjusted again so that a maximum amplitude of the convergence corrected voltage will be made larger. PA1 modified V-blanking pulse generating means for generating a modified V-blanking pulse having a pulse width which becomes larger as an amplitude of the V-deflection voltage becomes larger, PA1 modified H-blanking pulse generating means for generating a modified H-blanking pulse having a pulse width which becomes larger as an amplitude of the H-deflection voltage becomes larger, and PA1 convergence signal generating and correcting means for generating V- and H-convergence signals in accordance with each of the modified V- and H-blanking pulses and executing convergence correction for the CRT by means of said V- and H-convergence signals.